Sub-carrier spacing in LTE application framework

[JeffreyYeh]

Hello everyone

I'm trying to increase LTE subcarrier spacing from 15K to 30KHz with same bandwidth in application framework, which is 20MHz

Can I simply increase sub-carrier spacing by using 1024 FFT/IFFT instead of 2048 but keep 30.72MHz sample rate for 20MHz bandwidth LTE?

Since there are no other bandwidth mode except 20MHz

Anything do I have to notice when modifying FFT/IFFT VI in LTE application framework?

[ctarver]

Hi,

I'm not with NI, but I have some thoughts, since I've worked with making bandwidth modifications to the LTE design.

If you modify the subcarrier spacing and keep other things constant, I imagine you'll need to change the number of subcarriers per RB. In the FPGA design, there is a block that makes counters for symbol, subcarrier, subframe, RB, called "Index Generator." You'll need to modify that so that you don't screw up your timing. One example of where it can be found is in the DL TX loop of the DPGA design > DL TX IQ Processing > DL TX Resource Mapper. 

Also, I imagine that the size of a transport block will change. This could mess up the PDSCH channel, so you may need to make some modifications to this in the "PDSCH Config Calc" block. 

Regards,

ctarver

[JeffreyYeh]

Thanks, ctarver

I assume after modifying FFT/IFFT size to 1024 with 30.72 MHz sample rate

I'll get 600 occupied sub-carriers, 1 dc and 423 guard sub-carriers just like 10 MHz LTE but evenly distributed in 20 MHz

In this case, need to change the number of PRB from 100 to 50, it seems only sequence lens or structure lens has to change

In other cases, if I want to keep channel coding, processing most part unchanged, I'll have to modify bandwidth to 40 MHz by changing sampling rate and maybe has to change lots of VI between RF and base-band signal (DDC, DUC, FIR... etc)

Which seems more probably things will go wrong, so I'm going to try modifying PRB number and resource allocation first

[JeffreyYeh]

I try to increase sub-carrier spacing by lower the frequency resolution

After modifying Index generator, everything about # of PRBs, # of sub-carriers and FFT/IFFT VIs

Now I'm stock at the place between FFT/IFFT and DDC/DUC

In LTE AF for 20 MHz TX, there are 30720 samples per tti after 2048 IFFT

With 1024 IFFT only 15360 samples per tti will be generated

I'm wondering about can DUC work functionally with the same sample rate (30.72 MHz) but half the size of data